In transmitting and receiving the signals modulated in one or another manner, a very important characteristic is the demodulation threshold, i.e., the ratio of the signal power to the noise power (signal-to-noise ratio, SNR), at which the carrier wave of the signal being received ceases to be derived, which results in loss of the reception. The demodulation threshold depends essentially on the demodulation type employed at the transmission side, and the noiseless coding type.
One of possible techniques for lowering the demodulation threshold consists in enlarging a spectrum of the signal being transmitted using the so called pseudo-random sequences (PRS). Particularly, the use of the signals like the PRS is directly specified in radio communication systems operating in those frequency ranges which do not require for licensing the right of exclusive use of one or another part thereof (i.e., in the ranges of 2.400 to 2.483 MHz and of 5.725 to 5.850 MHz). Beside this, requirements for using a frequency separation, for limiting a radiation power (maximum 100 mW in any direction), and for employing normalized frequency band of the radiated signal depending on the PRS base. The PRS base is a repetition cycle thereof expressed in the length intervals of one element of the PRS. For the above ranges, the PRS base should be not less than 10.
In the modems produced at the present time and intended for operating in the above frequency ranges, the modulation of the type QPSK is used, which PRS base is equal to 15.
In these modems which sensitivity ranges from −90 to −98 dBm, the carrier and clock synchronization takes place at the signal-to-noise ratio from 0 to +3 dB, which is caused by a non-linear synchronization circuit. Should the synchronization circuit in these modems is linear, the sensitivity thereof could be improved by 7-10 dB.
Known are various proposals for increasing the demodulation threshold.
Thus, the Japan Laid-open Application No. 2001-237908 (2001 Aug. 31) discloses a system for extracting a synchronization signal from the QAM signal, which system ensures a quasi-synchronous detection. The U.S. Pat. Nos. 6,717,462 (2004 Apr. 6) and 6,727,772 (2004 Apr. 27) disclose methods and systems for transmitting and receiving the QAM signals with a carrier frequency adjustment. However, both these patents provide only a simple processing of the common QAM signal, which does not permit to lower the demodulation threshold.
The US Patent Applications Nos. 2004/0022328 (2004 Feb. 5) and 2005/0111601 (2005 May 26) disclose systems and methods for transmitting quadrature-amplitude modulation (QAM) signals, where the receiver synchronization is based on determining a rotation angle of the received signal vector in the phase space of the complex coordinates. However, these systems employ non-linear techniques of synchronization.
The Japan Laid-open Applications Nos. 2005-117366 (2005 Apr. 28), 2005-217636 (2005 Aug. 11) and 2006-262494 (2006 Sep. 28) disclose QAM methods and systems, where additional specific symbols are introduced for the synchronization at the transmitting side, and the synchronization adjustment is performed at the receiving side using these symbols. Similar principle is used in the International Application WO 2006/135275 (2006 Dec. 21). However, the use of the additional symbols complicates the processing of the received signal.